1. Field of the Invention
The present invention relates to a laser diode driving circuit and, more particularly, to a laser diode driving circuit which prevents the overshoot of a light emission power and which can adjust the light emission time to be equal to the preset time.
2. Description of the Related Art
A laser diode is utilized as a light source for image formation in an electrophotographic printer, a light source of an optical transmitter in optical communication, and in other various fields.
FIG. 9 shows the structure of a laser diode driving circuit. The reference numeral 3a represents a laser diode, 11 an inverter for inverting the polarity of a laser diode light emission signal (laser diode driving signal) LDS, 12 a first NPN switching transistor which is turned on/off in accordance with an inversion signal *LDS, 13 a second switching transistor which is differentially turned on/off with respect to the first switching transistor 12 in accordance with the laser diode light emission signal LDS, 14 an impedance element provided with an impedance characteristic which is approximately equivalent to that of the laser diode 3a, 15 a constant-current source for supplying a current to the laser diode 3a and the impedance element 14, and 16, 17 resistors. The laser diode 3a is disposed between the collector of the first switching transistor 12 and the constant-current source 15, and the impedance element 14 is disposed between the collector of the second switching transistor 13 and the constant-current source 15.
The laser diode driving circuit has the above-described differential structure, and it is so controlled that the sum of the currents Id and Ii flowing to the transistors 12 and 13, respectively, is constant. When the level of the laser diode light emission signal LDS is lowered so as to emit light from the laser diode 3a, the first switching transistor 12 is energized, while the second switching transistor 13 is turned off. As a result, the driving current Id flows to the laser diode 3a, while the current Ii becomes zero, and the laser diode 3a emits light. On the other hand, when the level of the laser diode light emission signal LDS is raised so that the light of the laser diode 3a is extinct, the first switching transistor 12 is turned off, while the second switching transistor 13 is energized. As a result, the current Ii flows to the impedance element 14, while the driving current Id becomes zero, and the light is extinct. In this case, the aptitude to overshoot at the rise of the driving current Id is cancelled by the aptitude to undershoot of the impedance element 14, so that a driving current wave form having little overshoot is obtained.
In this way, the differential structure can reduce the overshoot and the undershoot of the driving current which are caused in a laser diode driving circuit having no differential structure, and can produce a comparatively stable amount of light emission.
In a conventional laser diode driving circuit, the inverter 11 inverts the laser diode light emission signal LDS, and the switching transistors 12, 13 are differentially turned on/off in accordance with the complementary signals LDS, *LDS.
However, since the inversion of the polarity takes time, there is a delay in the inversion signal *LDS, which delay produces a lag of current switching, i.e., a lag of switching timing between the first and the second transistors. Due to the lag of timing, the differential function does not work sufficiently, and overshoot is produced in the driving current Id and, hence, in the light emission power. That is, in a conventional laser diode driving circuit, reduction in overshoot and undershoot is not satisfactory.
In addition, in a conventional laser diode driving circuit, the light emission time becomes shorter than the preset time due to the lag of switching timing, so that it is impossible to obtain the desired light emission power in total.
FIG. 10 shows the signal wave form of each part of the laser diode driving circuit so as to explain the shortened light emission time.
The inversion signal *LDS (b) rises or falls a predetermined time td behind the time for falling and the time for rising of the laser diode light emission signal LDS (a).
When only the first switching transistor 12 is conductive, the driving current Id which is necessary for light emission flows to the laser diode 3a. Therefore, the laser diode 3a starts light emission by the rise of the inversion signal *LDS and the light is extinct at the rise of the laser diode light emission signal LDS. As a result, the laser diode light emission time (c) is (T-td), which is shorter by td than the preset time T.